(1) Field of the Invention
The present invention relates generally to a system for automatically controlling a vehicle speed to a desired cruise speed and a method therefor. The present invention particularly relates to the system for automatically controlling the vehicle speed to the desired cruise speed and method therefor, the system control function being inhibited when a system for controlling traction operates to suppress a slip on a drive wheel.
(2) Background of the Art
Recently, both control systems are installed together in the same vehicle, i.e., a system for automatically controlling a vehicle speed to a desired cruise speed which is effective during e.g., the vehicle travel on a freeway without operation of an accelerator pedal and a system for suppressing slips of drive wheels during travel oh a road surface having a very low friction coefficient such as a road covered with snow or ice are installed together in a vehicle.
Such individual control systems are exemplified respectively by a Japanese Patent Application First Publications (Tokkai) sho 60-4428 published on Jan. 10, 1985 and sho 61-35945 published on June 23, 1986.
The traction control systems are also exemplified by U.S. Pat. Applications Ser. No. 918,137 filed on Oct. 14, 1986. Ser. No. 918,080 filed on Oct. 14, 1986, now U.S. Pat. No. 4,763,912, and Ser. No. 918,081 filed on Oct. 14, 1986, now U.S. Pat. No. 4,711,850. The automatic vehicle speed controlling systems are also exemplified by U.S. Pat. Applications Ser. No. 057,086 filed on June 3, 1987, now U.S. Pat. No. 4,829,438, No. 109,031 filed on Oct. 16, 1987, now U.S. Pat. No. 4,845,622; No. 130,473 filed on Jan. 21, 1988, No. 146,558 filed on Jan. 21, 1988, No. 143,092 filed on Jan. 12, 1988, No. 061,295 filed on June 12, 1987, now U.S. Pat. No. 4,870,584; No. 055,516 filed on May 28, 1987, now U.S. Pat. No. 4,835,696 and No. 169,218 filed on Mar. 16, 1988.
In the vehicle in which the previously proposed automatically vehicle speed controlling system is mounted even if an accelerator pedal is placed in a fully released position (no depression of the accelerator pedal), an opening angle of a first throttle valve installed in a vehicular engine as a driving force adjusting mechanism is adjusted in accordance with a control variable outputted to a first throttle actuator so that a vehicle speed is controlled so as to coincide with a set cruise speed.
A cruise speed controller is mounted for receiving a signal derived from a tire wheel (drive wheel) speed sensor detecting a tire wheel speed of a drive wheel and outputting an acceleration/deceleration command to the actuator of the first throttle valve so that the drive wheel speed coincides with a target drive wheel speed at which the vehicle driver desires to cruise (set cruise speed). For example, if the drive wheel speed is below the target drive wheel speed, the opening angle of the first throttle valve is increased so that the vehicle driving force is accordingly increased and drive wheel speed is increased.
The cruise speed controller then starts its control function in response an actuation of a cruise set switch installed in a vehicle compartment and, on the other hand, when the set switch is released and/or a brake or a clutch pedal is depressed, the cruise control function of the cruise speed controller is immediately inhibited and released.
In addition, a second throttle valve is installed downstream of the first throttle valve and is normally placed in a fully open position. The second throttle valve is actuated by means of a second throttle actuator.
A traction controller is installed for receiving a signal derived from a sensor detecting a vehicle body speed (approximately, a driven wheel speed), a signal derived from the tire wheel speed sensor, and a signal derived from a throttle opening angle sensor detecting the opening angle of the first throttle valve , determining whether a slip occurs on the basis of the determination whether a difference between both drive wheel and vehicle body speeds is above a thresold level, and for outputting a command signal to the second actuator to close the second throttle valve in a direction toward which the second throttle valve is fully closed so that the driving force is decreased. Since the slip occurs in a case where a drive wheel torque exceeds a frictional force against a road surface, the driving force is suppressed so as not to exceed the frictional force against the road surface.
However, in a vehicle in which both systems described above are mounted, since both the cruise speed controller and the traction controller carry out independent controls for the driving force from each other, both controls are often interfere with each other under certain running conditions.
FIGS. 5 (A) to 5 (C) illustrate characteristic graphs caused by the previously proposed systems when the vehicle enters on a road surface of an ascending slope having a gradient of, e.g., 3% and having a low friction coefficient, e.g., .mu.=0.1 with the cruise controller operated. As shown in
FIGS. 5(a) to 5(C) the drive wheel speed and vehicle body speed deviate from each other due to the occurrence of slip during an interval of time from approximately 10 seconds to approximately 30 seconds after the run on the above-described road surface. Therefore, the traction controller operates in response to the deviation of both drive wheel and vehicle body speeds so as to largely close the second throttle valve so that the drive wheel torque does not exceed the road surface frictional force.
On the other hand, since the tire wheel speed becomes reduced below the target vehicle speed in FIGS. 5(A) to 5(C)) due to the reduced driving force, the cruise controller is operated to open the first throttle valve so as to return the drive wheel speed to the target vehicle speed.
In the method described above, interference occurs due to the opening of the one throttle valve and closing of the other throttle valve. For example, the slip can pe suppressed by a slight closure of the second throttle valve in a case where the first throttle valve is largely closed. However, in a case where the first throttle valve is largely open, the second throttle valve needs to be largely closed to reduce the driving force to the same magnitude. In addition, since the vehicle runs win a transient state immediately after the vehicle enters on the low frictional road surface from a normal high frictional road surface the it is difficult to accurately determine an opening angle through which the throttle valve is closed in good response, with a relationship to the control speed of the traction controller taken into consideration, and the effect of slip suppression may be deteriorated.
Hence, the traction control needs to have a higher priority than the cruise control when the vehicle runs on a low frictional road surface on which a steering force of a steering wheel when the vehicle steering wheel is operated is lost and on which a steering holding force midway through the steering operation is lost. In addition, it is desirable to temporarily halt the operation of the cruise controller when the vehicle runs on such a low frictional road surface in order not to impair the effect of the traction control.
Since the intake air quantity is determined according to a position at which a cross sectional area of an air flow passage becomes minimum by means of either of the pair of throttle valves in such a case where the two throttle valves are disposed in series with each other, the control by means of the traction controller after the wide close of the second throttle valve is predominated. Therefore, although the control of the cruise speed controller can be neglected, the unncessary operation of the first throttle valve provides a cause of increase of wear-out of a sliding portion of the first throttle valve.